Originally posted by Racer X 69 Pentax K5IIs, Sigma 300mm f2.8 EX APO DG, Sigma 2.0x EX APO DG Tele Converter and Sigma 1.4x EX APO DG Tele Converter.
So with both teleconverters mounted, how is the focal length calculated?
I normally think of it in the order they mount, from memory you have the 1.4 on the lens itself and then the 2x attached to the camera body. so (300 x 1.4) x 2 = 840mm (FF) , As with Aperature, 2.8 minus 1 stop, goes to F4 minus 2 stops goes to F8
I have done this myself for a few moon photos, but quality really starting to fade .
Arrrr
cmohr